1. Field of the Invention
The present invention relates to a reversible heat-sensitive recording medium capable of reversibly recording and erasing an image by the application of heat.
2. Description of the Related Art
Japanese Patent early Publication Nos. 154198/1980 and 257883/1987 suggest reversible heat-sensitive recording media, according to which due to the difference of the temperatures upon heating, the transparency after being cooled can be reversibly changed, whereby images can be reversibly recorded thereon and erased therefrom. These recording media have a layered structure, including a layer, in which an organic low molecular substance such as a higher fatty acid is dispersed in an organic macromolecular resin such as a vinyl chloride-vinyl acetate copolymer in a fine particle form, as a heat-sensitive recording layer provided on a support.
The relation between the transparent state of these reversible recording media and the heat history given therefor is shown in FIG. 1. FIG. 1 is a constructional diagram showing the relation between the temperature of reversible heat-sensitive recording media and the transparency. The opaque (milky white state) portion can become transparent by heating it to a given temperature in the range of T1 to T2, and then gradually cooling it down to room temperature TR [(1).fwdarw.(3).fwdarw.(5).fwdarw.(6)]. Conversely, in order to turn a transparent portion into an opaque state, the transparent portion may be heated to a temperature exceeding T3, and then gradually cooled down to room temperature [(6).fwdarw.(5).fwdarw.(4).fwdarw.(2).fwdarw.(1)]. In these cases, the transparent state or opaque state is stably maintained at room temperature TR as is.
In the conventional process, the range of the heating temperature, T1-T2, for an opaque portion becoming transparent (temperature range to achieve transparent state: TW) is very narrow and cannot be controlled in a desired range. Consequently, for a reversible heat-sensitive recording medium in an .opaque state becoming transparent, it is required to control the temperature strictly and, thus, the practical use of the conventional process entails great difficulty.
If a reversible heat-sensitive recording medium is heated for a sufficient duration of time by using a heating medium having a sufficient heat capacity, e.g., an oven or a heat block, in the case where the reversible heat-sensitive recording medium in an opaque state is to become transparent, the temperature range to achieve transparent state: TW (referred to as clarifying temperature" hereinafter) may not be so wide. However, if the heating media for the medium becoming opaque has an insufficient heat capacity, or if it cannot be heated for a sufficient period, e.g., in the case of the heat application by means of a thermal head or laser for a period of several milliseconds, then the clarifying temperature range TW should be set sufficiently broad, because a temperature gradient toward the thickness direction of the reversible heat-sensitive recording medium is caused upon being heated, resulting in the medium not being able to be heated to a uniform temperature. Nevertheless, the conventional reversible heat-sensitive media have a very wide range of the clarifying temperature, thus, it is difficult to use a thermal head, etc. as a heating medium for erasing images.
Yet, when a white opaque portion is erased to allow the media to become transparent by means of a heating medium which has an insufficient heat capacity or which cannot be heated over a sufficient period of time, the conventional reversible heat-sensitive media have the problem of not having sufficient erasing ability.